1. Field of the Invention
The present invention relates generally to transparent conductive layers (TCL) used for flat panel displays, solar panels, emissive backlight constructions, and the like.
2. General Background and State of the Art
A liquid crystal display (LCD) is a type of flat panel display used in various electronic devices. Generally, LCDs comprise two sheets of polarizing material with a liquid crystal solution therebetween. Each sheet of polarizing material typically comprises a substrate of glass or transparent plastic; a liquid crystal (LC) is used as optical switches. The substrates are usually manufactured with transparent electrodes, typically made of indium tin oxide (ITO) or another conductive metallic layer, in which electrical “driving” signals are coupled. The driving signals induce an electric field which can cause a phase change or state change in the LC material, the LC exhibiting different light-reflecting characteristics according to its phase and/or state.
Liquid crystals can be nematic, smectic, or cholesteric depending upon the arrangement of the molecules. A twisted nematic cell is made up of two bounding plates (usually glass slides or plastic plates), each with a transparent conductive coating (such as ITO or another conductor) that acts as an electrode, spacers to control the cell gap, two cross polarizers (the polarizer and the analyzer), and nematic liquid crystal material. Twisted nematic displays rotate the direction of the liquid crystal by 90°. Super-twisted nematic displays employ up to a 270° rotation. This extra rotation gives the crystal a much deeper voltage-brightest response, also widens the angle at which the display can be viewed before losing much contrast. Cholesteric liquid crystal (CLC) displays are normally reflective (meaning no backlight is needed) and can function without the use of polarizing films or a color filter. “Cholesteric” means the type of liquid crystal having finer pitch than that of twisted nematic and super-twisted nematic. Sometimes it is called “chiral nematic” because cholesteric liquid crystal is normally obtained by adding chiral agents to host nematic liquid crystals. Cholesteric liquid crystals may be used to provide bistable and multistable displays that, due to their non-volatile “memory” characteristic, do not require a continuous driving circuit to maintain a display image, thereby significantly reducing power consumption. Ferroelectric liquid crystals (FLCs) use liquid crystal substances that have chiral molecules in a smectic C type of arrangement because the spiral nature of these molecules allows the microsecond switching response time that makes FLCs particularly suited to advance displays. Surface-stabilized feroelectric liquid crystals (SSFLCs) apply controlled pressure through the use of a glass plate, suppressing the spiral of the molecules to make the switching even more rapid.
Some known LCD devices include chemically-etched transparent, conductive layers overlying a glass substrate. See, e.g., U.S. Pat. No. 5,667,853 to Fukuyoshi et al., incorporated herein by reference. Unfortunately, chemical etching processes are often difficult to control, especially for plastic films. Such processes are also not well-suited for production of the films in a continuous, roll-to-roll manner, on plastic substrates.
There are alternative display technologies to LCDs that can be used, for example, in flat panel displays. A notable example is organic or polymer light-emitting devices (OLEDs) or (PLEDs), which are comprised of several layers in which one of the layers is comprised of an organic material that can be made to electroluminesce by applying a voltage across the device. An OLED device is typically a laminate formed in a substrate such as glass or a plastic polymer. A light-emitting layer of a luminescent organic solid, as well as adjacent semiconductor layers, are sandwiched between an anode and a cathode. The semiconductor layers can be hole-injecting and electron-injecting layers. PLEDs can be considered a subspecies of OLEDs in which the luminescent organic material is a polymer. The light-emitting layers may be selected from any of a multitude of light-emitting organic solids, e.g., polymers that are suitably fluorescent or chemiluminescent organic compounds. Such compounds and polymers include metal ion salts of 8-hydroxyquinolate, trivalent metal quinolate complexes, trivalent metal bridged quinolate complexes, Schiff-based divalent metal complexes, tin (IV) metal complexes, metal acetylacetonate complexes, metal bidenate ligand complexes incorporating organic ligands, such as 2-picolylketones, 2-quinaldylketones, or 2-(o-phenoxy) pyridine ketones, bisphosphonates, divalent metal maleonitriledithiolate complexes, molecular charge transfer complexes, rare earth mixed chelates, (5-hydroxy) quinoxaline metal complexes, aluminum tris-quinolates, and polymers such as poly(p-phenylenevinylene), poly(dialkoxyphenylenevinylene), poly(thiophene), poly(fluorene), poly(phenylene), poly(phenylacetylene), poly(aniline), poly(3-alkylthiophene), poly(3-octylthiophene), and poly(N-vinylcarbazole). When a potential difference is applied across the cathode and anode, electrons from the electron-injecting layer and holes from the hole-injecting layer are injected into the light-emitting layer; they recombine, emitting light. OLEDs and PLEDs are described in the following United States patents, all of which are incorporated herein by this reference: U.S. Pat. No. 5,707,745 to Forrest et al., U.S. Pat. No. 5,721,160 to Forrest et al., U.S. Pat. No. 5,757,026 to Forrest et al., U.S. Pat. No. 5,834,893 to Bulovic et al., U.S. Pat. No. 5,861,219 to Thompson et al., U.S. Pat. No. 5,904,916 to Tang et al., U.S. Pat. No. 5,986,401 to Thompson et al., U.S. Pat. No. 5,998,803 to Forrest et al., U.S. Pat. No. 6,013,538 to Burrows et al., U.S. Pat. No. 6,046,543 to Bulovic et al., U.S. Pat. No. 6,048,573 to Tang et al., U.S. Pat. No. 6,048,630 to Burrows et al., U.S. Pat. No. 6,066,357 to Tang et al., U.S. Pat. No. 6,125,226 to Forrest et al., U.S. Pat. No. 6,137,223 to Hung et al., U.S. Pat. No. 6,242,115 to Thompson et al., and U.S. Pat. No. 6,274,980 to Burrows et al.
In a typical matrix-address light-emitting display device, numerous light-emitting devices are formed on a single substrate and arranged in groups in a regular grid pattern. Activation may be by rows and columns, or in an active matrix with individual cathode and anode paths. OLEDs are often manufactured by first depositing a transparent electrode on the substrate, and patterning the same into electrode portions. The organic layer(s) is then deposited over the transparent electrode. A metallic electrode can be formed over the electrode layers. For example, in U.S. Pat. No. 5,703,436 to Forrest et al., incorporated herein by reference, transparent indium tin oxide (ITO) is used as the hole-injecting electrode, and a Mg—Ag-ITO electrode layer is used for electron injection.
Previous methods of manufacturing such films have not succeeded in manufacturing such films by a continuous process on flexible substrates, yielding films with desirable properties such as high transmittance, low electrical resistance, and stability to temperature and humidity.
For example, PCT Publication No. WO 99/36261, by Choi et al. (Polaroid Corp.) published on Jul. 22, 1999, and incorporated by this reference, describes the deposition of ITO/Au/Ag/Au/ITO multilayered films on polymer (Arton substrate). In this multilayered structure, the Ag layer has a thickness of 10–15 nm and the two ITO layers have a thickness of 35–50 nm. As compared with ITO/Ag/ITO multilayered films, an Au/Ag/Au sandwiched layer works as a conductive layer in the multilayered structure and exhibits an enhanced corrosion resistance as the 1–1.5 nm Au layers prevent the water or oxygen from entering the Au/Ag interfacial area. It was reported that the ITO/Au/Ag/Au/ITO films have a sheet resistance below 10 Ω/square and a transmittance above 80%. However, the deposition process for these multilayered films is much more complicated than the deposition process for ITO/Ag/ITO films.
U.S. Pat. No. 5,667,853 to Fukuyoshi et al., incorporated herein by reference, describes the formation of InCeO/Ag/InCeO films in which the indium cerium oxide (InCeO or “ICO”) layers have a thickness of about 35–50 nm and the Ag layer has a thickness of about 10–15 nm. The InCeO films were deposited by sputtering a target that was formed by doping 10–30% CeO2 into In2O3. The cerium can effectively block the diffusion of oxygen atoms from the InCeO films to the InCeO/Ag interfacial layer. On the other hand, the Ag layer actually contains 1 atom percent Au and 0.5 atom percent Cu to enhance the stability of Ag atoms in the Ag layer. The design of the chemical compositions in both the InCeO and the Ag layers was reported to effectively improve the structural stability of the InCeO/Ag/InCeO films. The InCeO/Ag/InCeO films exhibit a low sheet resistance of 3–5 Ω/square and a high optical transmittance of above 90%. The deposition of InCeO/Ag/InCeO films was also disclosed in U.S. Pat. No. 6,249,082 to Fukuyoshi et al., incorporated by this reference. However, the deposition of these films was only performed on a rigid glass substrate. The invention was not applied to the actual manufacture of information displays.
Other methods for producing such films and the films thus produced are described in U.S. Pat. No. 4,166,876 to Chiba et al., U.S. Pat. No. 4,234,654 to Teijin, U.S. Pat. No. 4,345,000 to Kawazoe et al., U.S. Pat. No. 4,451,525 to Kawazoe et al., U.S. Pat. No. 4,936,964 to Nakamura, U.S. Pat. No. 5,178,957 to Kolpe et al., U.S. Pat. No. 6,171,663 to Hanada et al., U.S. Published patent Application No. US 2001/0050222 by Choi et al., PCT Patent Publication No. WO 98/12596 by Staral et al., European Patent Publication No. EP 1041644 by Cheung, and European Patent Publication No. EP 1155818, all of which are incorporated herein by this reference.
The technology disclosed in U.S. Pat. No. 5,667,853 employs silver (Ag) or a silver-gold alloy (Ag/Au) sandwiched between two layers of indium cerium oxide (InCeO). The Ag or Ag/Au alloy is susceptible to oxidation when exposed to air and in the presence of water and electrical voltage. Under these conditions, the metal tends to diffuse. This can cause electrical short circuits or visible stains, which damages the appearance of the display. The use of the InCeO layers is an improvement, making the Ag or Ag/Au construction much more stable and making the total layer transparent. However, for the purposes of flat panel displays, the electrode needs to be patterned by etching. The edged edge exposes the Ag or Ag/Au metal. Unless the metal is protected in some manner, the oxidation or corrosion might start from this location. Therefore, there is a need for a more stable construction for flat panel displays.
Moreover, Furukawa Metal (Tokyo, Japan) developed an alloy of silver which resist corrosion even after immersion in a salt water for 24 hours. The Alloy (APC) is Silver with small amount of Palladium and Copper (Silver 98%, Palladium 0.5% and copper 1.5%). This material is currently used for metallizing the digital video disc (DVD) or Comapct disc (CD) for improving reflectivity. This application require oxidation resistance for the life of DVD or CD. The use of this alloy for combination with the oxide layers to provide the optical transparency and to improves the stability of transparent conductive film even in a harsh condition of wet etching.
Accordingly, there is still a need for an improved construction for flat panel displays that avoids oxidation and corrosion.